You can put in an aftermarket head unit and just remove the factory amp. You can also try and find a used one from a junk yard, or eBay, or whatever. You can also put in the factory low-end head unit that drives the speakers directly, and remove the amp.
Though really, if you are tearing into the wiring, might as well go with a good aftermarket unit, IMO. The only reason I still have a GM headunit is because it interfaces with my stock rear seat DVD and stock rear seat audio.